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Rod-like brazed diamond tool fabricated by supersonic-frequency induction brazing with Cu-based brazing alloy
Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Rod-like brazed diamond tool fabricated by supersonic-frequency induction brazing with Cu-based brazing alloy Bojiang Ma ⁎, Qian Pang, Jianpeng Lou College of Electromechanical Engineering, Qingdao University of Science & Technology, 266061 Qingdao, PR China
a r t i c l e
i n f o
Article history: Received 5 September 2013 Accepted 10 October 2013 Available online 6 November 2013 Keywords: CuSnTi brazing alloy Diamond Induction brazing Rod-like diamond tool
a b s t r a c t Supersonic-frequency induction heating was used to fabricate a rod-like diamond tool brazed with a Cu-based brazing alloy in order to control the dropping of liquid brazing alloys that takes place in other brazing technologies. The brazing alloy can be spread evenly and the diamond grits can be correctly exposed on the rod-like brazed diamond tool by using a proper induction brazing technique. Dissolution, diffusion, and chemical recombination took place between the diamond grits and the brazing alloy, and a diffusion band approximately 30 μm wide was observed between the brazing alloy and the steel substrate. This demonstrates that metallurgical bonding occurred both at the interface of the diamond grits and the brazing alloy and at the interface of the brazing alloy and the steel substrate. A boring test of the brazed diamond drill showed that the total grain fracture percentage of the diamond grits was higher than the falloff percentage. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The advantage of induction brazing is convenience and a short brazing time, and diamond grits brazed by induction heating exhibit minimal thermal damage such as cracks and graphitization of the diamond [1,2]. However, carbide is difficult to form at the interface of diamonds and the brazing alloy if the brazing time is short as in other brazing technologies. Liquid brazing alloys can wet diamond well only through carbide, so liquid brazing alloys exhibit poor wetting of diamond in short times of noninduction brazing technology. In induction heating, active elements can diffuse rapidly in the liquid brazing alloy owing to the stirring effect of the electromagnetic force [3]. The rapid diffusion of the active elements can promote the formation of carbide, thus improving the wetting of brazing alloys toward diamond. The gap between the periphery of the workpiece and the internal surface of the inductor is generally uneven, and thus the temperature surrounding the workpiece surface is uneven [4]. While the brazing alloy may melt and flow at high temperature points, it cannot melt at low temperature points yet. The temperature on the workpiece surface will be made to be even through the revolving round the axis of the workpiece. A Cu-based brazing alloy has a low melting temperature of approximately 900 °C, and thus, the liquid alloy easily spreads and runs over a steel substrate [5,6]. In vacuum furnace brazing, a Cu-based brazing alloy is only fit to be put on the end face of wheel that can be put horizontally. Thus, a Cu-based brazing alloy cannot be used to fabricate a rod-like brazed diamond tool such as a diamond drill because the liquid ⁎ Corresponding author. Tel./fax: +86 532 88956068. E-mail address: [email protected] (B. Ma). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.10.017
alloy always flows down, making it difficult to braze diamond grits on the steel rod substrate. In induction brazing, the liquid brazing alloy on the revolving workpiece is subjected to centrifugal force, surface tension, gravity, and an electromagnetic force, all acting in different directions. The centrifugal force can be controlled by adjusting the rotation rate and the electromagnetic force can be controlled by adjusting the induction current. The combination of these operations can easily prevent the dropping of the liquid alloy, and thereby, effective brazing could be realized. The wear modes of diamond grits on the brazed diamond drill during use have their own characteristics owing to the relatively low hardness of the Cu-based brazing alloy. The wear modes of the diamond grits were also researched in our experiment. 2. Experimental Diamond grits of a 50/60 mesh were used in the experiment, with a 1045 steel rod (diameter: 8 mm) used as the substrate. One end of the rod was ground into a spherical cap of 4-millimeter radius. CuSnTi alloy powder (200/300 mesh) – composed of 74 wt.% Cu, 16 wt.% Sn, and 10 wt.% Ti – was used as the brazing alloy. A binder is made up of 5 vol.% acrylic adhesive and 95 vol.% dimethylbenzene. A CuSnTi brazing alloy layer 130 μm thick (315 mg) was stuck to the first 14 mm from the top of the spherical cap using the binder, followed by pre-coating with 122 diamond grits. The distance between the diamond grit particles was approximately 1 mm. A supersonic-frequency induction-heating power supply (power: 16 kW; frequency range: 15–35 kHz) was used as the heating source. The inductor was made of four circles of red-copper loops (inner diameter: 40 mm). A schematic of the induction heating system is shown in Fig. 1. Argon entered the quartz tube (7) through the intake
26
B. Ma et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
1 8 2 7 3 6
9
4 5
Fig. 1. Induction brazing device. 1: intake pipe, 2: iron clamp, 3: inductor, 4: coupling, 5: motor, 6: workpiece, 7: quartz tube, 8: plug with a through-hole, and 9: iron stand.
pipe (1). The temperature of the workpiece was measured through the quartz tube with an infrared radiation thermometer (range: 0–1500 °C). The motor (5) drove the workpiece (6) to rotate via the coupling (4) at a rotating speed of 15 rps (15 s− 1) in order to heat the workpiece evenly. The heated part of the workpiece was placed within the lower part of the inductor. The output current of the inductor was 450 A. The workpiece was heated to 950 °C for 10 s. A section specimen of the brazed diamond drill was prepared, and the composition diffusion between the brazed diamond, brazing alloy, and substrate was studied using an energy spectrometer (EDX, INCAPenta-FET × 3, Oxford Instruments, UK). The brazing diamond drill was then eroded with chloroazotic acid. The brazed diamond grits were cleansed with acetone and observed using a scanning electron microscope (S-4800, Hitachi-High Technologies, Japan). The ground and polished brazing alloy–substrate was etched using a solution comprising 10 mL of distilled water, 10 mL of nitric acid (1.4 g/cm3), and 10 mL of hydrochloric acid (1.19 g/cm3). Its metallurgical structure was observed using a metallographic microscope (XJZ-6A, Nanjing Jiangnan Yongxin Optics Co., LTD., P. R. China) and its interfacial composition was tested using an energy spectrometer (EDX, INCAPenta-FET × 3, Oxford Instruments, UK). The hardness of the brazing alloy–substrate was measured using a Vickers hardness tester with a load of 50 g for 10 s. A boring test was carried out using a remade upright driller [power of main motor: 4 kW; main shaft speed: 560 rpm (9.3 s−1)]. Ceramic tiles (6 mm thick, Shore hardness: 85) were used as the testing material. The feeding force was 300 N. The damage morphology of the diamond grits on the brazed diamond drill was observed using a scanning electron microscope (S-4800, Hitachi-High Technologies, Japan) and the number of various damage modes of the diamond grits was counted using a stereoscopic microscope (JSZ4, Nanjing Jiangnan Yongxin Optics Co., LTD., P. R. China).
Fig. 2. Brazed diamond drill.
circulates the liquid brazing alloy, which contributes to the diffusion of the elements in the liquid brazing alloy. The microstructure of the interface between the brazed diamond grits and the brazing alloy is shown in Fig. 3(a). The figure shows close bonding and an absence of cracks between the two layers. Line
3. Results and discussion 3.1. Microstructure of the brazed diamond drill The diamond drill brazed by induction brazing with a CuSnTi brazing alloy is shown in Fig. 2. The brazing alloy is well spread on the substrate and no brazing alloy sags are observed. The brazed diamond grits are well defined. The application of an alternating electromagnetic field leads to an alternating current in the brazing alloy owing to electromagnetic induction. Their interaction produces an electromagnetic force in the skin layer that is directed upward and at an angle from the drill axis [7]. The electromagnetic force counteracts the centrifugal and gravitational forces, thereby preventing sag in the liquid brazing alloy. The force also
Fig. 3. Microstructure and compositions at the interface of the brazed diamond grits and brazing alloy: (a) interface microstructure; and (b) line-scanning energy spectrum along the marked line in (a).
B. Ma et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
Fig. 4. Surface morphology of the brazed diamond.
scanning of the energy spectrum was carried out from the brazed diamond grits to the brazing alloy along the marked line shown in Fig. 3(a), and the results of the line scanning are shown in Fig. 3(b). Titanium gathered and carbon gradually changed near the interface. That is, a mutual diffusion of the elements took place between the brazed diamond grits and brazing alloy. The diffusion depth is several microns, thus demonstrating that metallurgical bonding was achieved between the brazed diamond grits and the brazing alloy. The surface morphology of the brazed diamond grits obtained was observed through a scanning electron microscope (Fig. 4) after the brazed diamond drill was eroded with a strong acid. It can be seen that an acicular compound was formed on the surface of the brazed diamond grits. An analysis of the energy spectra showed that the compound was composed of titanium and carbon. As shown in Figs. 3 and 4, dissolution of the alloy, element diffusion, and chemical recombination took place near the interface of the brazed diamond grits and brazing alloy. In other words, metallurgical bonding took place at the interface of the brazed diamond grits and brazing
27
alloy, and this metallurgical bonding ensured the adequate holding of the diamond grits in the brazing alloy. The microstructure at the interface of the CuSnTi brazing alloy and the substrate is shown in Fig. 5(a). It can be seen that there are three layers, the brazing alloy layer, transition zone, and substrate, and the width of the transition band is approximately 30 μm. Fig. 5(b) displays the energy spectrum of the transition band shown in Fig. 5(a). Elements of both the brazing alloy (Cu, Sn, and Ti) and the substrate (Fe) are apparent in the transition band. That is, the transition band is the result of the mutual diffusion of elements between the brazing alloy and substrate. This mutual diffusion reinforced the wetting of the liquid brazing alloy on the substrate while also reinforcing the adhesion of the substrate to the liquid brazing alloy. The additive effects of the adhesive force and the electromagnetic force further counteracted the dropping of the liquid brazing alloy and enabled the brazing alloy to spread evenly on the substrate. It is thus clear that firm bonding between the brazing alloy and steel substrate can be realized through supersonic-frequency induction heating, in spite of the short brazing time. Many massive phases dispersed in the brazing alloy layer are seen from Fig. 5(a). The irregular massive phases are mainly composed of intermetallic compounds of copper and titanium (Cu3Ti and CuTi3) or that of tin and titanium (Sn3Ti5) [8]. The intermetallic compounds are all hard phases. The compounds dispersed in a soft base (copper) improve the impact resistance and wear resistance of the brazing alloy. The values of micro-hardness (Vickers hardness) nearby the interface between the CuSnTi brazing alloy and the substrate are shown in Fig. 6. The Vickers hardness significantly increased in the transition zone. The average Vickers hardness of the Cu-based brazing alloy layer was about 3350 MPa, indicating that the alloy can meet general processing demands. 3.2. Wear characteristics of diamond grits on brazed diamond drill As shown in Fig. 7, there were four kinds of wear modes of the diamond grits on the brazed diamond drill: wear with abrasion platform, tip broken, whole-grain fracture (WGF), and pullout. The first
Fig. 5. Interface of Cu-based brazing alloy and substrate: (a) microstructure; and (b) energy spectrum of diffusion band.
28
B. Ma et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
Fig. 6. Vickers hardness of substrate–transition band–brazing alloy.
two are fair wear and tear and the second two are unfair wear and tear. The unfair wear and tear no longer presents cutting ability. A great deal of unfair wear and tear can give rise to a high cutting force acting on the diamond grits of a brazed diamond drill, resulting in an increase in the wear speed of the diamond grits significantly shortening the life of the brazed diamond drill [9,10]. Counting the percentages of the different wear modes is important in terms of understanding the wastage of the diamond grits at certain stages while boring holes with a brazed diamond drill. In the boring test, the whole-grain fracture and pullout of diamond grits from the brazed diamond drill were counted for every six holes bored (shown in Fig. 8). The brazed diamond drill can no longer be used when the drill shakes and makes a harsh grinding noise while in use. As shown in Fig. 8, twenty-four holes could be bored using the brazed diamond drill fabricated in this experiment. During boring, the percentage of whole-grain fracture of diamond grits was higher than that of pullout, which implies the strong hold the brazing alloy had on the diamond grits.
Fig. 8. Percentages of WGF and pullout of diamond grits from brazed diamond drill.
4. Conclusions Supersonic-frequency induction heating was used to fabricate a rodlike diamond tool brazed with a Cu-based brazing alloy. Proper brazing process parameters kept the liquid brazing alloy forces balanced, preventing dropping of the liquid brazing alloy. The resulting brazing alloy was thus well spread on the substrate and the brazed diamond grits were well defined. Observation of the microstructure showed that there was metallurgical bonding at the interface of the diamond grits and brazing alloy, which ensured adequate holding of the diamond grits by the brazing alloy. The transition zone between the brazing alloy and substrate was approximately 30 μm wide. This transition zone reinforced the wetting of the liquid brazing alloy toward the substrate while simultaneously reinforcing the adhesion of the substrate to the liquid brazing alloy. The results of the boring test showed that the percentage of pullout of the diamond grits from the brazed diamond drill was much lower than that of the whole-grain fracture of the diamond grits.
Fig. 7. Wear modes of diamond grits on brazed diamond drill: (a) wear with abrasion platform; (b) tip broken; (c) whole grain fracture; and (d) pullout.
B. Ma et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
Acknowledgments We would like to acknowledge the financial support of the Natural Science Foundation of Shandong Province of P. R. China (Grant No. ZR2010EM015).
References [1] Huang SF, Tsai HL, Lin ST. Effects of brazing route and brazing alloy on the interfacial structure between diamond and bonding matrix. Mater Chem Phys 2004;84:251–8. [2] Ma BJ, Yu QX. Hot-filament chemical vapor deposition of amorphous carbon film on diamond grits and induction brazing of the diamond grits. Appl Surf Sci 2012;258:4750–5. [3] Volz MP, Mazuruk K. Flow transitions in a rotating magnetic field. Exp Fluids 1996;20(6):454–9.
29
[4] Huang MS, Huang YL. Effect of multi-layered induction coils on efficiency and uniformity of surface heating. Int J Heat Mass Transfer 2010;53:2414–23. [5] Acharya NN. Thermal analysis of slow cooled copper–tin alloys. J Mater Sci 2001;36:4779–95. [6] Lavrinenko LA, Evdokimov VA, Mel'nikova VA, Naidich YV. Contact interaction and the wetting of graphite and diamond by melts of CuSnNiTi system. Powder Metall Met Ceram 1998;37:512–6. [7] Deng AY, Wang EG, Xu YY, Zhang XW, He JC. Experimental research on melting surface behavior in mold under compound magnetic field. Acta Metall Sinica 2010;46:1018–24. [8] Wang J, Liu CL, Leinenbach C, Klotz UE, Uggowitzer PJ, Löffler JF. Experimental investigation and thermodynamic assessment of the Cu–Sn–Ti ternary system. Calphad 2011;35:82–94. [9] Hwang TW, Evans CJ, Malkin S. High speed grinding of silicon nitride with electroplated diamond wheels, part 1: wear and wheel life. J Manuf Sci Eng (Trans ASME) 2000;122:32–40. [10] Hwang TW, Evans CJ, Malkin S. High speed grinding of silicon nitride with electroplated diamond wheels, part 2: wheel topography and grinding mechanisms. J Manuf Sci Eng (Trans ASME) 2000;122:42–50.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Rod-like brazed diamond tool fabricated by supersonic-frequency induction brazing with Cu-based brazing alloy Bojiang Ma ⁎, Qian Pang, Jianpeng Lou College of Electromechanical Engineering, Qingdao University of Science & Technology, 266061 Qingdao, PR China
a r t i c l e
i n f o
Article history: Received 5 September 2013 Accepted 10 October 2013 Available online 6 November 2013 Keywords: CuSnTi brazing alloy Diamond Induction brazing Rod-like diamond tool
a b s t r a c t Supersonic-frequency induction heating was used to fabricate a rod-like diamond tool brazed with a Cu-based brazing alloy in order to control the dropping of liquid brazing alloys that takes place in other brazing technologies. The brazing alloy can be spread evenly and the diamond grits can be correctly exposed on the rod-like brazed diamond tool by using a proper induction brazing technique. Dissolution, diffusion, and chemical recombination took place between the diamond grits and the brazing alloy, and a diffusion band approximately 30 μm wide was observed between the brazing alloy and the steel substrate. This demonstrates that metallurgical bonding occurred both at the interface of the diamond grits and the brazing alloy and at the interface of the brazing alloy and the steel substrate. A boring test of the brazed diamond drill showed that the total grain fracture percentage of the diamond grits was higher than the falloff percentage. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The advantage of induction brazing is convenience and a short brazing time, and diamond grits brazed by induction heating exhibit minimal thermal damage such as cracks and graphitization of the diamond [1,2]. However, carbide is difficult to form at the interface of diamonds and the brazing alloy if the brazing time is short as in other brazing technologies. Liquid brazing alloys can wet diamond well only through carbide, so liquid brazing alloys exhibit poor wetting of diamond in short times of noninduction brazing technology. In induction heating, active elements can diffuse rapidly in the liquid brazing alloy owing to the stirring effect of the electromagnetic force [3]. The rapid diffusion of the active elements can promote the formation of carbide, thus improving the wetting of brazing alloys toward diamond. The gap between the periphery of the workpiece and the internal surface of the inductor is generally uneven, and thus the temperature surrounding the workpiece surface is uneven [4]. While the brazing alloy may melt and flow at high temperature points, it cannot melt at low temperature points yet. The temperature on the workpiece surface will be made to be even through the revolving round the axis of the workpiece. A Cu-based brazing alloy has a low melting temperature of approximately 900 °C, and thus, the liquid alloy easily spreads and runs over a steel substrate [5,6]. In vacuum furnace brazing, a Cu-based brazing alloy is only fit to be put on the end face of wheel that can be put horizontally. Thus, a Cu-based brazing alloy cannot be used to fabricate a rod-like brazed diamond tool such as a diamond drill because the liquid ⁎ Corresponding author. Tel./fax: +86 532 88956068. E-mail address: [email protected] (B. Ma). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.10.017
alloy always flows down, making it difficult to braze diamond grits on the steel rod substrate. In induction brazing, the liquid brazing alloy on the revolving workpiece is subjected to centrifugal force, surface tension, gravity, and an electromagnetic force, all acting in different directions. The centrifugal force can be controlled by adjusting the rotation rate and the electromagnetic force can be controlled by adjusting the induction current. The combination of these operations can easily prevent the dropping of the liquid alloy, and thereby, effective brazing could be realized. The wear modes of diamond grits on the brazed diamond drill during use have their own characteristics owing to the relatively low hardness of the Cu-based brazing alloy. The wear modes of the diamond grits were also researched in our experiment. 2. Experimental Diamond grits of a 50/60 mesh were used in the experiment, with a 1045 steel rod (diameter: 8 mm) used as the substrate. One end of the rod was ground into a spherical cap of 4-millimeter radius. CuSnTi alloy powder (200/300 mesh) – composed of 74 wt.% Cu, 16 wt.% Sn, and 10 wt.% Ti – was used as the brazing alloy. A binder is made up of 5 vol.% acrylic adhesive and 95 vol.% dimethylbenzene. A CuSnTi brazing alloy layer 130 μm thick (315 mg) was stuck to the first 14 mm from the top of the spherical cap using the binder, followed by pre-coating with 122 diamond grits. The distance between the diamond grit particles was approximately 1 mm. A supersonic-frequency induction-heating power supply (power: 16 kW; frequency range: 15–35 kHz) was used as the heating source. The inductor was made of four circles of red-copper loops (inner diameter: 40 mm). A schematic of the induction heating system is shown in Fig. 1. Argon entered the quartz tube (7) through the intake
26
B. Ma et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
1 8 2 7 3 6
9
4 5
Fig. 1. Induction brazing device. 1: intake pipe, 2: iron clamp, 3: inductor, 4: coupling, 5: motor, 6: workpiece, 7: quartz tube, 8: plug with a through-hole, and 9: iron stand.
pipe (1). The temperature of the workpiece was measured through the quartz tube with an infrared radiation thermometer (range: 0–1500 °C). The motor (5) drove the workpiece (6) to rotate via the coupling (4) at a rotating speed of 15 rps (15 s− 1) in order to heat the workpiece evenly. The heated part of the workpiece was placed within the lower part of the inductor. The output current of the inductor was 450 A. The workpiece was heated to 950 °C for 10 s. A section specimen of the brazed diamond drill was prepared, and the composition diffusion between the brazed diamond, brazing alloy, and substrate was studied using an energy spectrometer (EDX, INCAPenta-FET × 3, Oxford Instruments, UK). The brazing diamond drill was then eroded with chloroazotic acid. The brazed diamond grits were cleansed with acetone and observed using a scanning electron microscope (S-4800, Hitachi-High Technologies, Japan). The ground and polished brazing alloy–substrate was etched using a solution comprising 10 mL of distilled water, 10 mL of nitric acid (1.4 g/cm3), and 10 mL of hydrochloric acid (1.19 g/cm3). Its metallurgical structure was observed using a metallographic microscope (XJZ-6A, Nanjing Jiangnan Yongxin Optics Co., LTD., P. R. China) and its interfacial composition was tested using an energy spectrometer (EDX, INCAPenta-FET × 3, Oxford Instruments, UK). The hardness of the brazing alloy–substrate was measured using a Vickers hardness tester with a load of 50 g for 10 s. A boring test was carried out using a remade upright driller [power of main motor: 4 kW; main shaft speed: 560 rpm (9.3 s−1)]. Ceramic tiles (6 mm thick, Shore hardness: 85) were used as the testing material. The feeding force was 300 N. The damage morphology of the diamond grits on the brazed diamond drill was observed using a scanning electron microscope (S-4800, Hitachi-High Technologies, Japan) and the number of various damage modes of the diamond grits was counted using a stereoscopic microscope (JSZ4, Nanjing Jiangnan Yongxin Optics Co., LTD., P. R. China).
Fig. 2. Brazed diamond drill.
circulates the liquid brazing alloy, which contributes to the diffusion of the elements in the liquid brazing alloy. The microstructure of the interface between the brazed diamond grits and the brazing alloy is shown in Fig. 3(a). The figure shows close bonding and an absence of cracks between the two layers. Line
3. Results and discussion 3.1. Microstructure of the brazed diamond drill The diamond drill brazed by induction brazing with a CuSnTi brazing alloy is shown in Fig. 2. The brazing alloy is well spread on the substrate and no brazing alloy sags are observed. The brazed diamond grits are well defined. The application of an alternating electromagnetic field leads to an alternating current in the brazing alloy owing to electromagnetic induction. Their interaction produces an electromagnetic force in the skin layer that is directed upward and at an angle from the drill axis [7]. The electromagnetic force counteracts the centrifugal and gravitational forces, thereby preventing sag in the liquid brazing alloy. The force also
Fig. 3. Microstructure and compositions at the interface of the brazed diamond grits and brazing alloy: (a) interface microstructure; and (b) line-scanning energy spectrum along the marked line in (a).
B. Ma et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
Fig. 4. Surface morphology of the brazed diamond.
scanning of the energy spectrum was carried out from the brazed diamond grits to the brazing alloy along the marked line shown in Fig. 3(a), and the results of the line scanning are shown in Fig. 3(b). Titanium gathered and carbon gradually changed near the interface. That is, a mutual diffusion of the elements took place between the brazed diamond grits and brazing alloy. The diffusion depth is several microns, thus demonstrating that metallurgical bonding was achieved between the brazed diamond grits and the brazing alloy. The surface morphology of the brazed diamond grits obtained was observed through a scanning electron microscope (Fig. 4) after the brazed diamond drill was eroded with a strong acid. It can be seen that an acicular compound was formed on the surface of the brazed diamond grits. An analysis of the energy spectra showed that the compound was composed of titanium and carbon. As shown in Figs. 3 and 4, dissolution of the alloy, element diffusion, and chemical recombination took place near the interface of the brazed diamond grits and brazing alloy. In other words, metallurgical bonding took place at the interface of the brazed diamond grits and brazing
27
alloy, and this metallurgical bonding ensured the adequate holding of the diamond grits in the brazing alloy. The microstructure at the interface of the CuSnTi brazing alloy and the substrate is shown in Fig. 5(a). It can be seen that there are three layers, the brazing alloy layer, transition zone, and substrate, and the width of the transition band is approximately 30 μm. Fig. 5(b) displays the energy spectrum of the transition band shown in Fig. 5(a). Elements of both the brazing alloy (Cu, Sn, and Ti) and the substrate (Fe) are apparent in the transition band. That is, the transition band is the result of the mutual diffusion of elements between the brazing alloy and substrate. This mutual diffusion reinforced the wetting of the liquid brazing alloy on the substrate while also reinforcing the adhesion of the substrate to the liquid brazing alloy. The additive effects of the adhesive force and the electromagnetic force further counteracted the dropping of the liquid brazing alloy and enabled the brazing alloy to spread evenly on the substrate. It is thus clear that firm bonding between the brazing alloy and steel substrate can be realized through supersonic-frequency induction heating, in spite of the short brazing time. Many massive phases dispersed in the brazing alloy layer are seen from Fig. 5(a). The irregular massive phases are mainly composed of intermetallic compounds of copper and titanium (Cu3Ti and CuTi3) or that of tin and titanium (Sn3Ti5) [8]. The intermetallic compounds are all hard phases. The compounds dispersed in a soft base (copper) improve the impact resistance and wear resistance of the brazing alloy. The values of micro-hardness (Vickers hardness) nearby the interface between the CuSnTi brazing alloy and the substrate are shown in Fig. 6. The Vickers hardness significantly increased in the transition zone. The average Vickers hardness of the Cu-based brazing alloy layer was about 3350 MPa, indicating that the alloy can meet general processing demands. 3.2. Wear characteristics of diamond grits on brazed diamond drill As shown in Fig. 7, there were four kinds of wear modes of the diamond grits on the brazed diamond drill: wear with abrasion platform, tip broken, whole-grain fracture (WGF), and pullout. The first
Fig. 5. Interface of Cu-based brazing alloy and substrate: (a) microstructure; and (b) energy spectrum of diffusion band.
28
B. Ma et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
Fig. 6. Vickers hardness of substrate–transition band–brazing alloy.
two are fair wear and tear and the second two are unfair wear and tear. The unfair wear and tear no longer presents cutting ability. A great deal of unfair wear and tear can give rise to a high cutting force acting on the diamond grits of a brazed diamond drill, resulting in an increase in the wear speed of the diamond grits significantly shortening the life of the brazed diamond drill [9,10]. Counting the percentages of the different wear modes is important in terms of understanding the wastage of the diamond grits at certain stages while boring holes with a brazed diamond drill. In the boring test, the whole-grain fracture and pullout of diamond grits from the brazed diamond drill were counted for every six holes bored (shown in Fig. 8). The brazed diamond drill can no longer be used when the drill shakes and makes a harsh grinding noise while in use. As shown in Fig. 8, twenty-four holes could be bored using the brazed diamond drill fabricated in this experiment. During boring, the percentage of whole-grain fracture of diamond grits was higher than that of pullout, which implies the strong hold the brazing alloy had on the diamond grits.
Fig. 8. Percentages of WGF and pullout of diamond grits from brazed diamond drill.
4. Conclusions Supersonic-frequency induction heating was used to fabricate a rodlike diamond tool brazed with a Cu-based brazing alloy. Proper brazing process parameters kept the liquid brazing alloy forces balanced, preventing dropping of the liquid brazing alloy. The resulting brazing alloy was thus well spread on the substrate and the brazed diamond grits were well defined. Observation of the microstructure showed that there was metallurgical bonding at the interface of the diamond grits and brazing alloy, which ensured adequate holding of the diamond grits by the brazing alloy. The transition zone between the brazing alloy and substrate was approximately 30 μm wide. This transition zone reinforced the wetting of the liquid brazing alloy toward the substrate while simultaneously reinforcing the adhesion of the substrate to the liquid brazing alloy. The results of the boring test showed that the percentage of pullout of the diamond grits from the brazed diamond drill was much lower than that of the whole-grain fracture of the diamond grits.
Fig. 7. Wear modes of diamond grits on brazed diamond drill: (a) wear with abrasion platform; (b) tip broken; (c) whole grain fracture; and (d) pullout.
B. Ma et al. / Int. Journal of Refractory Metals and Hard Materials 43 (2014) 25–29
Acknowledgments We would like to acknowledge the financial support of the Natural Science Foundation of Shandong Province of P. R. China (Grant No. ZR2010EM015).
References [1] Huang SF, Tsai HL, Lin ST. Effects of brazing route and brazing alloy on the interfacial structure between diamond and bonding matrix. Mater Chem Phys 2004;84:251–8. [2] Ma BJ, Yu QX. Hot-filament chemical vapor deposition of amorphous carbon film on diamond grits and induction brazing of the diamond grits. Appl Surf Sci 2012;258:4750–5. [3] Volz MP, Mazuruk K. Flow transitions in a rotating magnetic field. Exp Fluids 1996;20(6):454–9.
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